1. Field of the Invention
The invention relates to a copy sheet, and in particular, a carbonless copy sheet that provides good product stability even when the sheet is exposed to a moist environment. The copy sheets of this invention are particularly useful in a wide variety of applications that require stability in severe weather conditions.
2. Description of Related Art
Carbonless copy forms are well known in the copying industry, and have been utilized in a wide variety of applications and environments. A typical carbonless copy form comprises at least two sheets arranged in juxtaposed contact with each other. A first sheet (typically called the "CB" or coated back sheet) comprises a substrate with a coating on its lower surface. The coating contains a binder with microcapsules containing a color former. A second sheet (typically called the "CF" or coated front sheet) comprises a substrate coated with a layer containing developer. The color former and developer are selected such that when they come into physical contact, a chemical reaction occurs that produces a distinct color.
When imaging pressure is exerted on the copy form by writing, typing or otherwise applying selective pressure, the microcapsules are ruptured in a selective pattern thereby releasing or transferring the color forming material in a corresponding pattern to the layer containing developer. A chemical reaction occurs and produces in the CF sheet a color image corresponding to the imaging pressure. If more than one copy is desired, it is conventional to add one or more intermediate sheets containing a color former coating on one side and a color developer layer on the other side. The sheets are arranged so that a color former coating on one sheet is placed in contact with a color developing layer on an adjacent sheet.
Manufacturers of carbonless copy forms have long sought a product that provided a stable image of high intensity. Images decompose or are otherwise damaged by exposure to sunlight or atmospheric moisture. The coatings containing the developer material must achieve a balance between being sufficiently porous to permit the color forming material to penetrate to a sufficient depth to create a sharp and dense image, yet not so porous that the image is unprotected from environmental factors that will diminish the image quality. If the surface of the coating containing the developer material is not porous enough, the image will not form. If the surface of the coating is too porous, the image is readily degraded. This balance has been particularly difficult to achieve where the form is to be used in adverse weather conditions.
CF coatings known in the prior art typically contain a water soluble binder or viscosity control additive that increases the attraction of the coating surface to moisture. It has been observed that the presence of atmospheric moisture is a major factor in the tendency of an image produced in a carbonless form to fade. Apparently, the action of oxygen or other oxidizing agents in degrading the chemical reaction product that creates the image is accelerated in a humid or moist environment.
There is a need for a carbonless copy sheet that produces a high intensity image that will be stable in a variety of weather conditions, particularly those involving exposure to excessive moisture. There is also a need for a sheet that can be used in a variety of outdoor applications such as hunting license tags, lumber tags, crop or game tags, luggage tags, tags for disabled vehicles, tags for crime scene investigations and the like.
The present inventors have found that the coated front sheet of the invention, when used in a carbonless copy form, has low affinity for water; reduces image fading, and retains dimensional stability even when exposed to humid and moist environments.